Development and pathogenicity analysis of full-length infectious cDNA clones of citrus yellow mottle-associated virus in citrus plants

Citrus yellow mottle-associated virus(CiYMaV)belonging to the subgenus Mandarivirus within the genus Potexvirus,was first identified in 2018 from Pakistan(CiYMaV-PK),where it is endemic in several regions.Here,three full-length cDNA clones(pCiYMaV-FL-1,pCiYMaV-FL-18,and pCiYMaV-FL-22)corresponding to the genomic RNA of CiYMaV were constructed and then agroinfiltrated on Chandler pummelo(Citrus grandis)seedlings using the vacuum infiltration method.All the inoculated plants developed severe vein yellowing,leaf mottling,and dwarfing symptoms by 40 days post-infiltration(dpi).The results of a direct tissue blot immunoassay and reverse transcription polymerase chain reaction detection showed 94.7–100%infection rates of pCiYMaV-FL at 60 dpi.Despite there being no observed difference among the three clones in the severity of symptom,pCiYMaV-FL-22 showed the highest accumulation levels of viral RNA and coat proteins.Moreover,pCiYMaV-FL-22 successfully infected seven other citrus varieties and induced symptoms in five of them.Transmission electron microscopy identified the presence of filamentous virus particles in extracts from systemic leaves of the plants infected with pCiYMaV-FL-22 at 6-months post-infiltration.Taken together,the results indicate that Koch's postulates were fulfilled for CiYMaV in citrus plants.This is the first report of full-length infectious cDNA clones of CiYMaV,and thus,the data provide a basis for further study of the molecular mechanisms of virus infection and the development of a viral vector to express foreign genes in citrus plants.

Distribution and Research Advances of Citrus tristeza virus

Citrus tristeza virus （CTV） is one of the most important causal agents of citrus diseases and exists as numerous strains. CTV is replicated in phloem cells of plants within the family Rutaceae and is transmitted by a few of aphid species. CTV epidemics have caused death of millions of citrus trees in many regions all over the world, where the sour orange （Citrus aurantium） was used as rootstock. Also the production of grapefruit （C. paradisi） and sweet orange （C. sinensis） has been affected by CTV strains. CTV gives uplift to three prominent syndromes, namely quick-decline （tristeza）, stempitting and seedling-yellows. The disease is graft-transmissible in nature but not seed-transmitted. However, the tristeza disease in most citrus groves was a man-made problem created by the desire of horticulturists to introduce cultivars from other citrus growing areas. The utmost importance of the disease called for review articles in numbers of plant protection, epidemiology books, citriculture and proceedings. This review collects the information with respects to disease history, distribution host range, virus isolates association, identification and detection, transmission and management; especially on the current status of CTV prevailing and controlling in Pakistan. It provides valuable information for CTV disease and its controlling approaches.